The present invention relates generally to polarizing beam splitters and more specifically to a polarizing beam splitter for use in a liquid crystal spatial light modulator display system.
In the display industry, there is continuous pressure to produce virtual display systems with improved contrast ratios. This is a particular concern for display systems in which a liquid crystal spatial light modulator is illuminated by a light source to produce the viewable image. In many configurations of this type of virtual display system, a polarizing beam splitter is used as a main component of the optics for directing light from the light source through the system. In cases where the viewing area of the display is in line with the light source, the efficiency at which the polarizing beam splitter directs light of different polarizations through the system is critical in determining the contrast ratio of the system. U.S. Pat. No. 5,596,451 issued on Jan. 21, 1997 and entitled MINIATURE IMAGE GENERATOR INCLUDING OPTICS ARRANGEMENT, which is incorporated herein by reference, describes in detail several embodiments of such a system. For illustrative purposes, a prior art system of this type will be briefly described herein with reference to FIG. 1.
FIG. 1 illustrates a virtual image display system indicated by reference numeral 10. System 10 includes a light source 12 and a polarizer 14 for directing polarized light into a polarizing beam splitter (PBS) 16 as indicated by arrow 18. In this example, the light indicated by arrow 18 represents S-polarized light. PBS 16 reflects S-polarized light 18 into a reflective, liquid crystal spatial light modulator (SLM) 20. SLM 20 modulates the light by controllably changing the polarization of the light directed into SLM 20 thereby forming a pattern of modulated light that is reflected back into PBS 16. This pattern of modulated light from SLM 20 includes both S-polarized light as indicated by arrow 22 and P-polarized light as indicated by arrow 24. The S-polarized portion of the pattern of modulated light indicated by arrow 22 is reflected by PBS 16 back toward the light source and wasted as indicated by arrow 26. The P-polarized portion of the pattern of modulated light indicated by arrow 24 is allowed to pass through PBS 16 as indicated by arrow 28.
System 10 further includes a quarter wave plate 30 and a reflective magnifier 32. The combination of quarter wave plate 30 and reflective magnifier 32 changes the P-polarized portion of the modulated light indicated by arrow 28 into S-polarized light and reflects it back into PBS 16 as indicated by arrow 34. Since the pattern of light indicated by arrow 34 is now S-polarized light, PBS 16 reflects this pattern of light into a viewing area 36 as indicated by arrow 38 allowing this light to be viewed by a viewer.
As illustrated in FIG. 1, system 10 forms a viewable image by using SLM 20 to modulate the polarization of the light directed into PBS 16. However, because this configuration positions light source 12 in direct line with viewing area 36, the contrast ratio of the system is reduced by any of the S-polarized light indicated by arrow 18 from light source 12 which leaks through PBS 16 as indicated by dashed arrow 40. For currently available polarizing beam splitters which are designed to work over a broad spectrum of light wavelengths and over a broad range of angles of incidence, this leakage indicated by arrow 40 can be substantial.
The above described problem of leakage of light through the polarizing beam splitter has proved to make it very difficult to provide a practical color display system with a high contrast ratio using the configuration described above. This is because conventional polarizing beams splitters designed to work over a wide spectrum of light wavelengths and over a broad range of angles of incidence are not 100 percent effective and allow a substantial amount of leakage through the polarizing beam splitter. Therefore, it is desirable to provide a more efficient polarizing beam splitter which reduces this leakage problem. The present invention provides a polarizing beam splitter that works over a wide spectrum of light wavelengths and over a broad range of angles of incidence while substantially reducing or eliminating the leakage of light through the polarizing beam splitter compared to conventional polarizing beam splitters.